I've done that trip at least three times in the past few years and can assure you it's a memorable experience. Take a first class sleeper. There are not so many places in the world where you can afford to go first class on an overnight train, so take advantage of this opportunity.

The fare was not much more than 1200THB (~40USD). You could easily spend more than that on a hotel, so you save some money right there. If you want the compartment for yourself, you have to buy two tickets. Be sure to book in advance.

The train leaves in the late afternoon. You roll through some very interesting sites as the train pulls out. You arrive not long after dawn so you get to watch the country wake up - more interesting sites.

Need further encouragement to take the train? In the past few years, how often have you fondly reminisced about a flight you took? When you get off that train, I can assure you that you'll be telling your friends of the amazing experience you had.